refactor(upload): unify file-drop pipeline across all list pages

[Papyszoo]

Summary

• Replaces hand-rolled UploadableGrid + per-feature drag handlers with a single canonical <UploadDropZone> wrapper backed by a shared useDragAndDrop hook (one source of truth for the file-drop affordance across the app)
• Adds bounded-concurrency uploads (4 workers) and a skip-clone Zustand update path so dropping many files no longer hangs the UI
• Wires panel-wide tab-drop: tabs can now be dropped anywhere on a side panel, not just the dock bar — file/tab drags coexist via MIME-type isolation

What changed

New canonical drop wrapper

• UploadDropZone.tsx/.css — single visual treatment (dashed primary-color border on .drag-over), routes drops to onFilesDropped, ignores tab drags at hook level

Migrated 5 list pages → UploadDropZone

• EnvironmentMapList, ModelGrid, SoundList, SpriteList, TextureSetList

Migrated 4 container tabs → UploadDropZone

• ContainerSpritesTab, ContainerSoundsTab, ContainerEnvironmentMapsTab, ContainerTextureSetsTab
• Drops the redundant local "Uploading..." spinner overlay (the global UploadProgressWindow already shows per-file progress)

Deletes

• UploadableGrid.tsx + UploadableGrid.css (replaced by UploadDropZone)
• Stale :hover:not(.drag-over) rules in EnvironmentMapList.css, ModelGrid.css, TextureSetGrid.css that were leaking drop styling into the idle hover state

Performance

• uploadProgressStore: per-item updates skip cloning unaffected batches (O(N²) → O(N))
• useFileUpload.uploadMultipleFiles: bounded concurrency pool of 4 workers replaces serial for loop
• Drag listeners: module-level shared window listeners with refcount, instead of one pair per useDragAndDrop instance

Panel-wide tab drop

• DockPanelContent attaches the existing tab-drop handlers to the .dock-content div
• File drags pass through (handlers gate on draggedTab); inner UploadDropZone calls stopPropagation so file drops never reach the tab-drop listener

Test plan

• npx jest — 247/247 tests passing across 21 suites
• npx vite build — clean build
• Manual: drop a single file on each list page (Models / EnvMaps / Sounds / Sprites / TextureSets) — uploads + global progress window appear
• Manual: drop many (10+) files at once — UI stays responsive, 4 parallel uploads visible
• Manual: drop image files inside container detail tabs (Sprites / Sounds / EnvMaps / TextureSets) — creates assets in the container
• Manual: drag a tab from one side panel to the other side panel's content area (not just the dock bar) — tab moves
• Manual: while dragging a file, hover over the dock content area on a tab without an upload zone — no spurious drop highlight
• Manual: confirm idle hover on grid items no longer shows the dashed drop border

🤖 Generated with Claude Code